1. Field of the Invention
The present invention relates to a magnetic recording tape, in particular to a polyester film suitable for producing a magnetic recording tape of a ferromagnetic metal thin film such as a digital video cassette tape and data storage tape, in which data can be stored and reproduced with an MR head. The present invention also relates to a magnetic recording tape produced using the polyester film, and to a digital recorder.
2. Description of the Related Art
Domestic digital video cassette tapes (referred to as DVC tapes hereinafter) that have been in practical use since 1995 comprise a thin magnetic metal layer of Co with a thickness of 6 to 7 μm provided by vacuum evaporation on a base film, and a diamond-like carbon film coated on the magnetic thin film. A DV mini-cassette has a recording capacity that can record SD (Standard Definition) images for one hour.
The DVC tapes were developed as the first domestic digital video cassette in the world, and have been commercially successful due to the following advantages:
a. large storage capacity despite their small size;
b. good image quality and tone quality for many years since signals are not deteriorated;
c. high image quality and high tone quality without suffering from noise; and
d. no deterioration of image quality after repeated dubbing.
A polyester film having a surface coated with fine particles and a polymer has been used as the base film of the DVC tapes (for example, Japanese Examined Patent Publication Nos. 63-57238 and 6-99579, and Japanese Unexamined Patent Application Publication No. 2002-50028).
However, the so-called MICRO-MV standard was introduced in the autumn of 2001 for use with lightweight and compact video cameras, wherein one hour of video recording is possible using a cassette having a volume as small as 30% of the DV mini-cassette. While this new video standard is based on the digital recording method using a vacuum evaporation tape, as in the DVC standard, the picture compression method was changed from the DV compression (DVC standard) to MPEG2 compression, and the tape width was reduced from 6.35 mm (DVC standard) to 3.8 mm. In addition, the minimum recording wavelength was reduced to 0.29 μm from 0.49 μm (DVC standard), and the track pitch was reduced to 5 μm from 10 μm (DVC standard) and 6.7 μm (DVLP standard), meaning that the recording density was significantly increased. Furthermore, the thickness of the magnetic layer was significantly reduced to 50 nm in the MICRO-MV tape as compared with the thickness of 160 to 220 nm in the DVC tape.
Such high density recording and reproduction were enabled because MR heads (magnetoresistive heads) used in hard disk drives were employed for reproduction. MR heads take advantage of a phenomenon in which the electrical resistance of a metal thin film formed on a MR head changes in accordance with the changes of the applying magnetic field.
However, since the metal thin film provided on the MR head is as thin as 20 nm, it is readily worn down by allowing it to travel in contact with the magnetic recording tape. Accordingly, when the MICRO-MV tape is produced by using a conventional film used as the DVC tape base film, there is a problem in that the MR head needs to be frequently replaced due to the extremely short service life of the MR head (a continuous reproduction time of about 100 hours).